Avoid confusing prototype methods for static methods

Correct: `Array.from()` (it’s a static method)
Incorrect: `Array.join()` (doesn’t exist; it’s a prototype method)

This patch uses the common `#` syntax to denote `.prototype.`.

snippets/observeMutations.md

@@ -3,7 +3,7 @@
 Returns a new MutationObserver and runs the provided callback for each mutation on the specified element.
 
 Use a [`MutationObserver`](https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver) to observe mutations on the given element.
-Use `Array.forEach()` to run the callback for each mutation that is observed.
+Use `Array.prototype.forEach()` to run the callback for each mutation that is observed.
 Omit the third argument, `options`, to use the default [options](https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver#MutationObserverInit) (all `true`).
 
 ```js